After hydrocarbons have been removed from the ground, the fluid stream (such as crude oil or natural gas) is transported from place to place via pipelines. It is desirable to know with accuracy the amount of fluid flowing in the stream, and particular accuracy is demanded when the fluid is changing hands, or “custody transfer.”
Meter proving methods “prove” the accuracy of flow meter measurements. FIG. 1 illustrates a system 10 for proving a turbine meter 12. A turbine meter, based on turning of a turbine-like structure within the fluid stream, generates electrical pulses (illustrated in FIG. 1) where each pulse is proportional to a flow volume, and the rate of pulses proportional to flow rate. A prover time is the time period defined by a prover element flowing first past an upstream detector 16 then a downstream detector 18 in prover 20 (the prover time also illustrated in FIG. 1). Signals from the upstream detector 16 and downstream detector 18 indicating prover time are collected at a processor 26. The processor 26 also collects pulses from signal line 14 and determines which pulses fall within the prover time. The number of pulses generated by the turbine meter 12 during the prover time is indicative of the volume measured by the meter during the prover time. By comparing the prover volume to the volume measured by the meter, the meter may be “proved.”
FIG. 2 illustrates another system 50 for proving an ultrasonic flow meter 52. By ultrasonic it is meant that ultrasonic signals are sent back and forth across the fluid stream, and based on various characteristics of the ultrasonic signals a fluid flow may be calculated. Ultrasonic meters generate flow rate data in batches where each batch comprises many sets of ultrasonic signals sent back and forth across the fluid, and thus where each batch spans a period of time (e.g., one second). The flow rate determined by the meter corresponds to an average flow rate over the batch time period rather than a flow rate at a particular point in time.
The American Petroleum Institute (API) requires proving by comparing a prover volume to a meter volume, with the meter volume determined from pulses. Conforming to this standard dictates that data from an ultrasonic flow meter be converted to pulses for purposes of proving. Such a conversion may be carried out in an internal processor 54, with the pulses supplied to the external processor 26 to prove the ultrasonic meter 52 as described above. However, the pulses created by an ultrasonic meter may be based on an average flow rate over a batch time period with the pulses created after the end of a batch time period. Thus, even though a particular pulse may fall within the proving time, the fluid flow that formed the basis of the pulse may have occurred before the beginning of the prover time because of the batch operation of the meter. Likewise, a value of fluid flow through the meter during the proving time may be converted to pulses that fall outside the proving time because of the batch nature of the meter. These, and possibly other, difficulties in proving flow meters, such as ultrasonic flow meters, create shortcomings in related art meter proving methods.